Bass frequencies non-directional?

I have issues with our new live sound setup. Our stage is 48' wide, and we have a JBL sub (2x18 Marquis series, I believe) on the floor at each end. While sound checking this weekend, I found that while standing at the board (at the back center of the room) or on either side, the sound was a good mix of lows, mids, and highs. However, in the center, from front row to about 10-12 rows back, all I was hearing was mids and highs, which sounded sort of like an AM radio station. The subs pointed straight out and didn't seem to be covering the front sections of seats in the middle. I asked the soundman if we should turn the subs in more, and he said it wouldn't matter, since bass is "non-directional". I'm not a sound engineer, but it sounded pretty "directional" to me! I would also think that you would want the best possible sound in the center at the front as well as the back. Anyone else experience this?

Bass is non-directional, but depending on how far away from the source you are, you might experience more or less bass depending on whether you're standing at a node or an anti-node. Bass (at the frequencies you're talking about) tends to be "felt" rather than "heard". More often than not, everything is too loud anyway, so who gives a damn about the direction from which it comes! ;>

I'm not an expert, but, from what I've read about Bose speaker systems, they say that subwoofer placement is not critical since bass frequencies are non-directional. I don't have an exact publication to quote though. The size and shape of the room have a lot to do with it too, I think.

Originally posted by JohnL I have issues with our new live sound setup. Our stage is 48' wide, and we have a JBL sub (2x18 Marquis series, I believe) on the floor at each end. While sound checking this weekend, I found that while standing at the board (at the back center of the room) or on either side, the sound was a good mix of lows, mids, and highs. However, in the center, from front row to about 10-12 rows back, all I was hearing was mids and highs, which sounded sort of like an AM radio station. The subs pointed straight out and didn't seem to be covering the front sections of seats in the middle. I asked the soundman if we should turn the subs in more, and he said it wouldn't matter, since bass is "non-directional". I'm not a sound engineer, but it sounded pretty "directional" to me! I would also think that you would want the best possible sound in the center at the front as well as the back. Anyone else experience this?

Click to expand...

It takes a surface about half a wavelength long in length and width to cause a sonic "shadow." The wavelengths of bass frequencies are very long in comparison to any type of speaker cabinet you're likely to use; at 100 Hz they are about 11 feet long. Also, compared to these wavelengths, the difference in the paths taveled by bass-frequency sound waves to your left and right ears is practically nil, so they tend to sound monaural. These are why bass frequencies are said to be non-directional.

But the situation you described makes me think that maybe one of those subs was out of polarity with the other, so that when one driver was moving forward, the other was moving backward, and vice-versa. This would cause cancellation of the bass frequencies directly in front (and in back), up the middle, as you described.